A laser converts input power into a very narrow, intense beam of coherent radiant energy at a single frequency. Military applications for lasers include range finding and target designators, including beam-rider guidance for "smart" ordinance. On the defensive side, laser warning devices can be located on or in close vicinity to potential military targets.
Lasers may operate either continuously or in a pulsed mode. High power lasers of the type used in military applications usually operate in pulsed mode due to input power and cooling requirements.
Some known laser warning devices are actuated by a rapid increase in incident radiant energy, indicative of a high power pulse.
Other known laser warning devices exploit the coherence of laser energy. One such approach is to use an interferometer which may be designed and constructed so as to provide interfering radiation beam components, such that intensity spikes and nulls are observed for coherent radiation of the design frequency but not for noncoherent radiation. Filtering of noncoherent radiation is particularly important considering the environment for laser warning devices. The intensity of background noncoherent radiation will vary widely from the black of night to the light of day, and may be subjected to temporary sharp increases such as momentary reflected sunlight and/or proximal detonations. One such interferometer which has been used for laser warning is known as a Fabry-Perot interferometer or etalon, consisting of two partially silvered mirrors at a fixed distance apart to produce multiple reflected interference spectra of high dispersion and resolution for selected frequencies. At such frequencies, constructive interference occurs between the mirrors and an intensity spike is observed at the side opposite the incident side when the spacing satisfies integral solutions of: ##EQU1## where: n'=the material index of refraction at the wavelength .lambda..sub.0 ;
h=the mirror spacing; PA1 .theta.'=the internal angle of reflection; and PA1 .phi.=the phase change upon reflection. PA1 However, for coherent signals, interference effects cause each etalon 12 step to have a different transmission, depending on the wavelength of the incident light, and on the etalon 12 thickness (excluding the steps). The angle of arrival of the coherent signal also has a small effect. Preferably, the etalon 12 is designed such that its mean optical thickness (2ndcos.theta..sub.n), where n is the etalon index of refraction, d its average thickness, and .theta..sub.n the angle of the light in the etalon 12, is substantially larger than the coherence length of incoherent background sources, while also being substantially less than a coherence length of coherent sources that it is desired to detect. Thus, the average etalon 12 thickness determines the coherent/incoherent discrimination ability of the etalon 12.
Devices representative of those using etalons are disclosed in the following U.S. patents: Crane, Jr. U.S. Pat. No. 3,824,018, issued Jul. 16, 1974; Ballard et al. U.S. Pat. No. 4,515,478, issued May 7, 1985; Siebert U.S. Pat. No. 5,151,585, issued Sep. 29, 1992.
More specifically the "Coherent Light Source Detector" of the Crane, Jr. patent uses an etalon which is "scanned" by angularly moving it about an axis perpendicular to its optical axis. In this way the effective thickness of the etalon is continuously changed for coherent radiation at a given angle of incidence, and intensity spikes should be observed for coherent radiation having a frequency in a predetermined range.
The Ballard et al. patent addresses the problem of widely varying background illumination and laser intensity for a coherent frequency of interest. The invention involves directing light passing through the interferometer onto an array of photosensitive elements, and analyzing the intensity detected by such individual elements, particularly by determining an average intensity, to detect the interference pattern or fringes.
The Siebert patent is directed to a system in which adjacent etalons of different thicknesses are disposed close to separate photo detectors, in combination with "modulating" the incident radiation by transverse movement of the stepped etalon structure with a back and forth oscillatory motion over the detector elements. As stated in the Siebert patent,
Thus the object of Siebert is similar to that of Crane, Jr., i.e., effecting a change in the effective thickness of the etalon with respect to an adjacent detector for the purpose of determining whether or not incident radiation is sufficiently coherent as to produce fringes.